A polymer electrolyte fuel cell (PEFC) can operate in a low temperature region, has high energy conversion efficiency, and can be started in a short period of time, and a system of the PEFC can be reduced in size and weight. The PEFC is therefore expected to find applications in a power source for an electric vehicle, a portable electric power source, and a residential cogeneration system.
In the PEFC, a cathode catalyst layer is temporarily exposed to a high potential (e.g., 0.8 V or more) during repeated start-stop operation. In this case, when water is decomposed by an action of a noble metal such as platinum (Pt) serving as a cathode catalyst to generate oxygen, a carbon material carrying the noble metal undergoes oxidation corrosion by the oxygen, resulting in decomposition and deterioration of a catalyst support. The deterioration of the catalyst support reduces power generation performance of the PEFC, promotes dissolution and sintering of the noble metal such as Pt, and causes an additional reduction in catalytic activity.
In view of the foregoing, there has been developed such a carbon material for carrying a catalyst as to have high durability even under a high potential condition, to allow a noble metal catalyst such as Pt to be carried with high efficiency in a highly dispersed state in the order of several nm, and to cause no aggregation of the noble metal catalyst and the like.
A highly crystalline carbon material such as graphite is excellent in terms of durability but is not preferred in terms of an amount of a catalyst to be carried. This is because the highly crystalline carbon material such as graphite has a small specific surface area. In general, therefore, a trade-off relationship is found between the durability of the carbon material for carrying a catalyst and the amount of a catalyst to be carried.
Conventionally, for example, Patent Document 1 describes that carbon black or activated carbon is subjected to a heating treatment at 1,800 to 2,500° C. to increase its degree of graphitization, thereby providing a highly crystalline carbon material having high crystallinity.
In addition, Patent Document 2 describes that graphitized carbon black having excellent corrosion resistance is obtained by a method subjecting a mixture containing carbon black and a graphitization-promoting substance (B, Si, Al, Fe, or other compounds containing those atoms) to a heating treatment at 2,000 to 2,500° C., or a method further performing an activating treatment before or after the heating treatment.
In addition, Patent Document 3 describes that a carbon-based catalyst support is subjected to electrochemical oxidation in advance to improve its durability. In addition, Patent Document 4 describes that a carbon material (carbon black or carbon nanotube) carrying a noble metal catalyst is subjected to a heat treatment at 300 to 1,200° C. under an inert gas atmosphere to suppress corrosion of the carbon material.
In addition, Patent Documents 5 and 6 describe that a carbon alloy fine particle doped with nitrogen atoms and/or boron atoms is obtained as a base material for an electrode catalyst for a fuel cell by subjecting a thermosetting resin containing a nitrogen compound to a heat treatment at 400 to 1,500° C.
In addition, Patent Document 7 describes that a carbon material carrying a noble metal particle is obtained by adding and mixing a metal compound (at least one of iron, cobalt, nickel, chromium, and manganese) to a raw material for generating non-graphitizable carbon (selected from the group consisting of a thermosetting resin including polyfurfuryl alcohol, a furan resin, or a phenolic resin, brown coal, cellulose, polyvinylidene chloride, and lignin) and then carbonizing the mixture by a heat treatment.